Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and tripler excited states is characterized. We find that pi conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet-singlet, S-0-->S-1, singlet-triplet, S-0-->T-1, and triplet-triplet, T-1-->T-n, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T-1, is strongly localized on a single phenylene ring while the S-1 and T-n states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang-Rhys analysis of the tripler emission spectrum in model systems. (C) 1996 American Institute of Physics.